When a valve of a fluid system is closed, the abrupt stop of the fluid flow creates a reverse flow in the system. The reverse flow causes pulsating high pressure shock waves or transients. The shock waves generate a banging noise and vibrations of the pipes of the fluid system. This is known as ‘water hammer’. The banging noise is undesirable, in particular in fluid systems where valves are repeatedly opened and closed. Furthermore, the vibrations may cause damage to the pipes and/or to other parts of the fluid system.
It is known that water hammer can be reduced by closing the valve slowly. However, in some cases it is not possible, or at least inconvenient or improper, to simply close the valve sufficiently slowly to prevent or reduce water hammering, e.g. because proper operation of the valve requires that it is closed with a certain speed.
WO 2008/154919 discloses a valve where a correspondence between opening degree of the expansion valve and mutual position of the first valve part and the second valve part may be defined by a geometry of the first valve part and/or a geometry of the second valve part. Such a geometry may be or comprise size and/or shapes of openings defined in the first and/or second valve part, size and/or shape of valve elements/valve seats formed on the first and/or second valve parts, and/or any other suitable geometry. WO 2008/154919 does not disclose the problem of water hammering.
U.S. Pat. No. 4,121,618 discloses a mining machine removes coal in the ordinary manner with a plurality of digger elements and conveys dislodged coal into a crusher element, for example. The material, once crushed, falls in the direction of arrows into slurry hopper. The material in slurry hopper is mixed with water and removed by a pump and discharged to an outlet line, where it is transported to the surface of the mine, for example. U.S. Pat. No. 4,121,618 furthermore discloses, that one of the basic problems with the system as above constructed is the excessive water hammer which developed when control valve was rapidly closed as was necessary in order to prevent overflow of water from hopper. U.S. Pat. No. 4,121,618 does not disclose the problem of water hammering in relation to refrigeration systems and specific solutions to preventing water hammering in refrigeration systems. The valve of U.S. Pat. No. 4,121,618 is suited for a mining machine, not for a refrigeration system being very different to a mining machine.
U.S. Pat. No. 5,983,937 discloses a flow control device which is capable of reducing water hammer. The flow control device comprises first to third openings formed stepwise in a rotor side wall portion of a flow control valve disposed in a hot water circuit. Third positions (a) and (b) are established between a first position in which a first communication passage for communication between inlet and outlet pipes of the flow control valve is fully closed and a second position in which the first communication passage is fully open. A duty control is performed so that the rotor reciprocates repeatedly among the above four positions. Thus, the flow control device has been modified in order to allow water hammering to be reduced.